As soon as I hit Cruel difficulty, I cant level my characters. The 5% exp death penalty makes it literally impossible to gain enough exp before dying and losing it again. Does my build just suck or do I have to just farm Normal in order to level up?
Yes I think the build is just quite weak. You might want to invest in some more defences (resists + life) on gear and in the skill tree but also decent damage helps (kill mobs before they kill you). I also recommend using the flasks plugin, the 2.0 one, not the one that comes with the bot by default.